// event
@EventHandler
public void onPlayerDeathEvent(PlayerDeathEvent event) {
    Player plr = event.getEntity();
    Player killer = plr.getKiller();

    if (killer != null) {
        //Bukkit.broadcastMessage("debug\nvictim: " + plr.getName() + " killer: " + killer.getName());

        if (killer.hasPermission("headdrop.candrop")) {
            ItemStack stack = genHead(plr);
            killer.getInventory().addItem(stack);
        }
    }
}

// genHead method
private static ItemStack genHead(Player plr) {
    Material type = Material.matchMaterial("PLAYER_HEAD");
    assert type != null;

    ItemStack skull = new ItemStack(type, 1);

    SkullMeta meta = (SkullMeta) skull.getItemMeta();
    assert meta != null;

    meta.setOwningPlayer(plr);
    meta.setDisplayName(plr.getName() + "'s head");

    skull.setItemMeta(meta);

    return skull;
}